Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752514AbbDDJ7w (ORCPT ); Sat, 4 Apr 2015 05:59:52 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:36235 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752215AbbDDJ7u (ORCPT ); Sat, 4 Apr 2015 05:59:50 -0400 Message-ID: <551FB5F5.5050906@linux.vnet.ibm.com> Date: Sat, 04 Apr 2015 15:29:17 +0530 From: Preeti U Murthy User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Jason Low , Morten Rasmussen CC: "peterz@infradead.org" , "mingo@kernel.org" , Daniel Lezcano , "riel@redhat.com" , "vincent.guittot@linaro.org" , "srikar@linux.vnet.ibm.com" , "pjt@google.com" , "benh@kernel.crashing.org" , "efault@gmx.de" , "linux-kernel@vger.kernel.org" , "iamjoonsoo.kim@lge.com" , "svaidy@linux.vnet.ibm.com" , "tim.c.chen@linux.intel.com" Subject: Re: sched: Improve load balancing in the presence of idle CPUs References: <1427741729.5694.24.camel@j-VirtualBox> <551A5CCE.70008@linux.vnet.ibm.com> <1427828056.2492.24.camel@j-VirtualBox> <551B9514.80701@linux.vnet.ibm.com> <20150401170418.GX18994@e105550-lin.cambridge.arm.com> <1427954347.2556.43.camel@j-VirtualBox> In-Reply-To: <1427954347.2556.43.camel@j-VirtualBox> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15040409-8236-0000-0000-00000A8DB41B Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4229 Lines: 118 On 04/02/2015 11:29 AM, Jason Low wrote: > On Wed, 2015-04-01 at 18:04 +0100, Morten Rasmussen wrote: >> On Wed, Apr 01, 2015 at 07:49:56AM +0100, Preeti U Murthy wrote: > >>> I am sorry I don't quite get this. Can you please elaborate? >> >> I think the scenario is that we are in nohz_idle_balance() and decide to >> bail out because we have pulled some tasks, but before leaving >> nohz_idle_balance() we want to check if more balancing is necessary >> using nohz_kick_needed() and potentially kick somebody to continue. > > Also, below is an example patch. > > (Without the conversion to idle_cpu(), the check for rq->idle_balance > would not be accurate anymore) > > --- > kernel/sched/fair.c | 17 ++++++++++------- > 1 files changed, 10 insertions(+), 7 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index fdae26e..7749a14 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -7620,6 +7620,8 @@ out: > } > > #ifdef CONFIG_NO_HZ_COMMON > +static inline bool nohz_kick_needed(struct rq *rq); > + > /* > * In CONFIG_NO_HZ_COMMON case, the idle balance kickee will do the > * rebalancing for all the cpus for whom scheduler ticks are stopped. > @@ -7629,6 +7631,7 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > int this_cpu = this_rq->cpu; > struct rq *rq; > int balance_cpu; > + bool done_balancing = false; > > if (idle != CPU_IDLE || > !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu))) > @@ -7644,7 +7647,7 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > * balancing owner will pick it up. > */ > if (need_resched()) > - break; > + goto end; > > rq = cpu_rq(balance_cpu); > > @@ -7663,9 +7666,12 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > if (time_after(this_rq->next_balance, rq->next_balance)) > this_rq->next_balance = rq->next_balance; > } > + done_balancing = true; > nohz.next_balance = this_rq->next_balance; > end: > clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)); > + if (!done_balancing && nohz_kick_needed(this_rq)) > + nohz_balancer_kick(); > } > > /* > @@ -7687,7 +7693,7 @@ static inline bool nohz_kick_needed(struct rq *rq) > int nr_busy, cpu = rq->cpu; > bool kick = false; > > - if (unlikely(rq->idle_balance)) > + if (unlikely(idle_cpu(cpu))) > return false; > > /* > @@ -7757,16 +7763,13 @@ static void run_rebalance_domains(struct softirq_action *h) > enum cpu_idle_type idle = this_rq->idle_balance ? > CPU_IDLE : CPU_NOT_IDLE; > > + rebalance_domains(this_rq, idle); > /* > * If this cpu has a pending nohz_balance_kick, then do the > * balancing on behalf of the other idle cpus whose ticks are > - * stopped. Do nohz_idle_balance *before* rebalance_domains to > - * give the idle cpus a chance to load balance. Else we may > - * load balance only within the local sched_domain hierarchy > - * and abort nohz_idle_balance altogether if we pull some load. > + * stopped. > */ > nohz_idle_balance(this_rq, idle); > - rebalance_domains(this_rq, idle); > } > > /* > Solution 1: As exists in the mainline Solution 2: nohz_idle_balance(); rebalance_domains() on the ILB CPU Solution 3: Above patch. I observe that Solution 3 is not as aggressive in spreading load as Solution 2. With Solution 2, the load gets spread within the first 3-4 seconds, while with Solution3, the load gets spread within the first 6-7 seconds. I think this is because, the above patch decides to further nohz_idle_load_balance() based on the load on the current ILB CPU which has most likely pulled just one task. This will abort further load balancing. However, Solution 3 is certainly better at spreading load than Solution 1. Wrt IPIs, I see that Solution 2 results in increase in the number of IPIs by around 2% over Solution 3, probably for the same reason that Morten pointed out. Regards Preeti U Murthy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/